1. Technical Field
The present invention relates to a wireless local area network (WLAN) and, more particularly, to a system and method for handoff using a hybrid network wherein a new access point is scanned in the handoff process of a mobile terminal so that the mobile terminal is connected to the WLAN.
2. Related Art
In recent years, the transmission rate of the wireless local area network (WLAN) has increased significantly. While a cellular network still has a throughput that is too low to provide multimedia service, the WLAN provides Internet access service at a rate of 54 Mbps. The WLAN is a competitive technique applicable to mobile communication environments because of its broad bandwidth and low cost features.
The WLAN includes a terminal desiring access to a network, and access points relaying the terminal. The access points are connected to a backbone network.
The terminal may be a battery-driven notebook computer or personal digital assistant (PDA), but is not necessarily limited to such devices.
To be sent to another network, an 802.11 network frame should be translated into another type of frame. The access point bridges the networks. The backbone network delivers frames to a destination of the access point. A representative backbone network is an Ethernet.
The terminal has a WLAN module for connection to the WLAN. The terminal is provided with data service via the WLAN access point. One access point serves only a limited area. If the terminal moves to a coverage area of another access point, handoff occurs. Connection of a terminal from one access point to another is called a handoff between access points (inter-AP handoff). This process is called re-association in the IEEE 802.11 standard. For handoff, the terminal has to scan a new access point, which consumes much time and causes service delay.
Such delay in scanning the access point must be solved for real-time multimedia service, such as voice-over-Internet protocol (VoIP).
The IEEE 802.11 standard for a WLAN defines WLAN handoff including scanning, authentication, and re-association processes.
When a terminal moves from one access point area to another, a mobile node scans all channels provided by several neighboring access points. The IEEE 802.11 standard introduced two scanning methods.
The first method is a passive scanning method. In this method, a mobile node successfully receives signals from all channels, and waits to receive a beacon message from a scanned access point. The mobile node then determines the access point. In this method, overhead is reduced but delay time is too long. To overcome such a drawback of the passive scanning method, the active scanning method was introduced.
When active scanning is used, a mobile node broadcasts a probe request to each channel, and waits to receive a response from the channel during a minimum channel time (MinChannelTime). The mobile node scans all channels, and receives all beacon messages or probe responses to connect to the most appropriate access point.
Subsequent to the scanning process, an authentication process is performed. The mobile node transmits an authentication request to the newly determined AP, and the new AP determines whether the mobile node has permission to access the network, and returns an authentication response to the mobile node.
In this respect, authentication is a process of recognizing the presence of a mobile node and approving the mobile node to utilize basic service provided by an access point. Since a wireless network cannot provide the same security that a wired network can, the wireless network needs an additional authentication routine for determining whether a user accessing a network has access permission.
Finally, a re-association process is performed. The re-association process is a process in which a mobile terminal terminates an association with a present AP, and associates with a new AP, and a re-association request is initialized by the mobile terminal. The new AP 110-1 transmits a handoff request to the old AP, and in response to the handoff request, the old AP provides a handoff response containing information needed for the re-association to the new AP. Operation between the old AP and the new AP is defined in Inter Access Point Protocol (IAPP). In response to the handoff response, the new AP sends a re-association response to the mobile node.
It is known that the above-mentioned scanning process is a primary cause of delay in the overall handoff process of the WLAN mobile node. A selective scanning method is used to reduce scanning time. However, the selective scanning method does not provide effective reduction of scanning time because the method involves a standby time for discovering an optimal channel, e.g., a minimum channel time (MinChanelTime) to a maximum channel time (MaxChannelTime).
Thus, there is need for a channel scanning method capable of significantly reducing delay time in the handoff process of a WLAN mobile terminal.